Gallium nitride is a material widely used in the construction of blue, violet and white light emitting diodes, blue laser diodes, ultraviolet detectors and high power microwave transistor devices.
Because of the actual and potential uses of gallium nitride in the manufacture of low energy consumption devices suitable for use in a wide range of applications, there is great interest in gallium nitride films.
Gallium nitride films can be grown in a number of different ways, including molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) processes. MOCVD is the deposition method of choice for achieving films of sufficient quality for LED production.
However, for growing gallium nitride films, the MOCVD process suffers from the disadvantage that it has to be operated at a temperature of approximately 1000° C. Only materials that are capable of withstanding the relatively high temperatures, such as synthetic sapphire, can be used with this process.
Remote plasma enhanced chemical vapour deposition (RPECVD) is another growth method that can be used for growing films of Group (III) metal nitrides. Where the film to be grown is gallium nitride, the RPECVD technique enables the use of a growth temperature of about 600° C. to about 680° C., which is considerably lower than the growth temperature of the MOCVD process and enables the reduction of equipment costs. Another advantage of the RPECVD process is that temperature sensitive substrate materials more closely lattice matched to GaN, such as zinc oxide, can be used.
While RPECVD, by virtue of the remoteness of the plasma source from the substrate, is widely believed to be a technique that avoids film damage from species generated in the plasma, the inventors have found that films grown by this method can suffer severe damage even from relatively low energy species (that is, less than 14.5 eV in the case where a nitrogen plasma is used). Although damage from ionised particles and high energy electrons is avoided when using RPECVD, as a result of a rapid decay in energy within a short distance of the plasma source, relatively low energy active neutral nitrogen species that arrive at the substrate can still impart damage if they possess greater energies than the Ga—N bond strength (which is 2.2 eV). This damage may be manifested by the loss of nitrogen atoms from the film, or by the dislodgement of gallium and nitrogen atoms from their preferred lattice sites with their subsequent incorporation at other non-preferred lattice sites.
There accordingly exists a need for a further reduction of the energy of the active neutral nitrogen species that reach the substrate when using the RPECVD growth technique.
Considerable work has also been done on crystal size and oxygen segregation in GaN films [1]; on the recrystallisation prospects of GaN using ZnO as a buffer layer [2], and on a detailed comparison of the characteristics of GaN grown on quartz and sapphire substrates [3]. Early polycrystalline material produced by a RPECVD process combined with a laser CVD process was comparable to early MBE material growth with unintentional doped n-type material being produced with room-temperature mobility of 100-200 cm2/Vs [4] and carrier concentration around 1016 cm−3.
In PCT/AU2003/000598 a process for manufacturing a gallium rich gallium nitride film is described. That process is operated at a growth temperature of from about 480° C. to about 900° C. and in an atmosphere in which the partial pressure of oxygen is less than 10−4 Torr. Although the very low partial pressure of oxygen in the process described in the aforementioned publication already contributes to the production of metal nitride films of improved quality, such low partial pressures of oxygen generally require a reduction in system pressure during growth to achieve a low oxygen partial pressure.
The conventional RPECVD process suffers from the disadvantage of oxygen contamination caused by oxygen remaining in the system after evacuation, even down to a base pressure of about 10−6 to 10−8 Torr, and by the release of oxygen atoms from the walls of quartz or alumina containment vessels and tubes that are used in this process for the containment of the plasma. This presents a problem in that such oxygen atoms are liable to be incorporated into the gallium nitride film, causing the film to have undesirable properties. Oxygen is a dopant in gallium nitride films but may also segregate at high levels during growth at the temperatures used for RPECVD. Where oxygen incorporation into the gallium nitride film is uncontrolled, its concentration may exceed levels that can be tolerated or that are desirable, depending on whether there is a need for a certain amount of oxygen incorporation or whether its presence, even at low concentrations, is undesirable. Even where the electron carrier concentration is low, the electrical conductivity of the film may be affected by the presence of oxygen due to auto-compensation mechanisms which can cause the electrical conductivity and electron mobility to be very low.
Oxygen contamination may also result in small crystal sizes or even the formation of amorphous gallium nitride under certain growth conditions. Having a low level of is background oxygen present during film growth allows dopant levels to be set to device specifications by controlled input of dopant gases during film growth. It also ensures that crystal size is not limited by oxygen segregation.
When the surface of a containment vessel or tube made of alumina, quartz or silica is bombarded with high energy nitrogen ions forming part of a nitrogen plasma such as that obtained when the RPECVD process is used, some of the chemically bound oxygen atoms in the surface of the containment vessel or tube are released or dislodged as a result of the high energy of the nitrogen ions. This may allow causing a chemical reaction to occur between the dangling bonds produced at the vessel surface and the nitrogen ions. This chemical reaction naturally depends on the type of plasma and the material of the containment vessel or tube. The reaction can be thought of as a type of displacement reaction wherein oxygen is removed from the structure of the vessel and replaced by nitrogen.
An investigation into the possibility of the passivation of quartz and alumina containment vessels and tubes was reported by Butcher, K S A et al, in Studies of the Plasma Related Oxygen Contamination of Gallium Nitride Grown by Remote Plasma Enhanced Chemical Vapour Deposition, Phys. Stat. Sol. (c) No 1, 156-160 (2002). In that article, the authors described a method for the conditioning of an alumina containment vessel or tube wherein the alumina containment vessel or tube is conditioned in a nitrogen or ammonia plasma, depending on the type of plasma required to be used subsequently, for a prolonged period of from about 24 hours to several days. Where an ammonia plasma is used to condition the vessel or tube, some alumina molecules on the surface of the vessel or tube are converted to alane (AlH3), an unstable species which decays rapidly in air to form alumina and hydrogen gas. Where a nitrogen plasma is used to condition the vessel or tube, some alumina molecules on the surface of the vessel or tube are converted to aluminium nitride (AlN), which limits the evolution of further oxygen bearing species. However, in an atmosphere of air the aluminium nitride layer is also converted, over a period of time, to alumina and volatile gas products such as hydrogen, so that the conditioning process has to be repeated every time before a gallium nitride film is grown. The aforementioned report by Butcher et al concluded that oxygen contamination of a gallium nitride film grown by using a quartz containment tube or microwave window, even if subjected to some preconditioning by passing a nitrogen plasma therethrough, was unavoidable. The same would be expected to apply to fused silica tubes, in view of the chemical similarity of fused silica and quartz. The reason for the perceived unsuitability of quartz and fused silica for passivation can be ascribed to the chemical reaction which is believed to take place between the high energy nitrogen ions and the silica, which can be simplified as follows:SiO2(solid)+N2(plasma)→SiO(gas)+N2O(gas)  (1)As can be seen from equation (1), both the reaction products are gaseous. These gaseous products are swept away by the nitrogen plasma so that more silica is exposed to the nitrogen plasma.
There is therefore a need for a method and apparatus for growing a film of gallium nitride, wherein the oxygen contamination of the gallium nitride film is minimised.
In a RPECVD system a film of metal nitride is grown under partial vacuum in a growth chamber, using a reaction mixture depositing the metal nitride from reactants such as ammonia (and/or nitrogen) and trimethylgallium. The film is grown on a disc shaped substrate that is located on a rotating ring. The substrate is heated from below by a stationary heater. A nitrogen plasma is generated remotely and fed to the growth chamber. In the case of molecular beam epitaxy (MBE) the pressure at which the metal nitride film is grown may be as low as 10−5 Torr, while for RPECVD the pressure may be about 0.1-10 Torr.
The substrate is positioned about 2 to 3 mm above the heater. Depending on the technology used, the growth temperature may be from about 900° C. to about 1000° C. or from about 500° C. to about 1000° C. However, to achieve a desired growth temperature of about 650° C. on the substrate, it is necessary for the heater to be operated at a considerably higher temperature so that heat can be radiated to the substrate from below. It is thus not unusual to have to operate the heater at a temperature of about 1400° C.
One type of conventional heater for use in heating the substrate comprises a heating element or filament made of tungsten or tantalum wire of about 0.5 mm in diameter, wound around a disc shaped ceramic base with notches in its periphery.
Because of the use of plasmas, the environment within which metal nitrides are grown is typically a reducing atmosphere containing atomic nitrogen, which is very harsh on materials of construction. WO2003/097532 describes a process for the manufacture of a gallium rich gallium nitride film, using a RPECVD process. It is hereby incorporated by reference. With the higher pressure in the growth chamber used in the process described is in WO2003/097532, the conditions are more severe. The aforementioned conventional heaters may even be damaged at a stage prior to growth when the growth system is conditioned to the operating temperature and harsh gaseous environment referred to above.
Conventional heaters comprising resistance filaments made of tantalum or tungsten are embrittled at the growth temperatures used when exposed to the gases used in these systems, which include reactive nitrogen species from the plasma and hydrogen from the metalorganics, and eventually break. Alternately, they can burn through when they short-circuit because of metal deposited between the adjacent loops or windings, from the metalorganic source gases or from the windings themselves which may undergo some evaporation. The resistance wire fails either because of metal embrittlement and expansion or because the metal evaporates and condenses between windings, causing short-circuiting and overloading of short-circuited windings. A more reliable heater than those heaters of which the heating elements are made of tantalum or tungsten is accordingly needed to perform metal nitride semiconductor growths using the MBE and RPECVD techniques.
Another type of conventional heater is described in U.S. Pat. No. 6,140,624. This heater includes a dielectric base made of pyrolytic boron nitride and a heating element of pyrolytic graphite superimposed on the dielectric base. U.S. Pat. No. 5,343,022 describes a similar heating unit composed of a dielectric base of boron nitride and a pyrolytic graphite heating element encapsulated therein.
In U.S. Pat. No. 4,777,022, an epitaxial heater apparatus and process are described. The heater comprises resistive windings located about a core comprising a hollow cylindrical tube portion made of boron nitride, pyrolytic boron nitride or pyrifolyte.
However, these heaters are very expensive because of the use of pyrolytic boron nitride and pyrolytic graphite which are manufactured at high temperatures using chemical vapour deposition technologies with appropriate masks to grow a base incorporating a heating element, layer by layer. As a consequence of the high cost, these heaters are uneconomical in the context of commercial metal nitride film manufacture using the RPECVD technique.
There accordingly exists a need for a cheaper heater that is capable of withstanding the harsh operating conditions encountered in an RPECVD growth system used for growing metal nitrides.